SAN FRANCISCO—The Asiana Airlines jet that crashed while landing at San Francisco International Airport, killing two Chinese teenagers and injuring 182 others, was descending at a dangerously slow speed and pilots apparently ran out of time to correct their approach, investigators said Sunday.

While stopping short of pinpointing pilot error as the likely cause of the fiery crash Saturday, National Transportation Safety Board Chairman Deborah Hersman's comments indicated investigators were focused primarily on why the...